Kohlrabi – August Plant of the Month

What could be more fun than growing an edible spaceship? This member of the cabbage family is one of the oddest looking vegetables around. With its large, edible, bulbous stem sitting underneath big, cabbage-like leaves, it looks like some alien spacecraft that’s landed by accident in the middle of the vegetable garden! But its enlarged stem has a sweet, mild flavor that has been likened to a cross between a radish and a cucumber. Kohlrabi can be enjoyed crisp and raw, steamed, stir-fried, or added to soups and stews. And even the leaves are edible; cook them as you would kale.

Varieties: The bulbous stem of the kohlrabi plant may be white, pale green, or purple depending on the variety. 'Early Purple Vienna' and 'Early White Vienna' are open-pollinated heirloom varieties. ‘Azur Star’ has violet purple skin and crisp white flesh.’ Kossak’ is a white-skinned variety that remains sweet and tender even when the stems get as large as 8 inches in diameter. Some of the newer hybrid varieties are more heat and cold tolerant than older varieties. Pale green 'Winner' and purple 'Kolibri' are heat tolerant selections that mature quickly.

Site: Full sun (at least 8 hours of direct sun/day).

Soil: Plant in fertile, well-drained soil enriched with plenty of compost or other organic material.

When to Plant: This fast-growing vegetable does best when the weather is cool. It can be grown as a spring crop and also makes a good fall crop in many parts of the country. In the spring, direct sow seeds about a month before the last spring frost date and make repeat sowings every week or two while the weather remains cool. You can also start seeds indoors 4-6 weeks before the spring frost date and set transplants in the garden when they are about 4 inches tall. For a fall crop, sow seeds directly in the garden 8 to 10 weeks before the first expected fall frost date. In warm winter areas (Zones 9 and 10), you can make repeat sowings during the fall for harvest in the winter and early spring. Kohlrabi will withstand light frost.

Planting: Plant in wide rows or beds, spacing the seeds 3 inches apart. After the seedlings are a couple of inches tall, thin to a final spacing of 6 to 8 inches. Set transplants 6 to 8 inches apart.

Culture: Be sure to keep the soil consistently moist. A layer of mulch around plants will conserve soil moisture and keep weeds down as well. Give your plants a dose of soluble fertilizer such as fish emulsion when they are about a month old.

Troubleshooting: Flea beetles, cabbage worms, and cabbage loopers are common insect pests. Covering plants with lightweight floating row covers is an easy and safe way to keep these leaf chewing insects away. Cabbage worms and loopers can also be controlled with the microbial insecticide Bt, which affects only caterpillars. Club root is a fungus disease that causes galls to form on plant roots. Maintaining a soil pH of 6.5 -6.8 will help to suppress this disease.

Harvesting: The best advice about harvesting kohlrabi is not to wait too long. Most varieties are ready for harvesting just 6 to 7 weeks from planting and are the most tender and flavorful when the bulbs are 2 to 4 inches in diameter. Fall crops that ripen in cool weather don't get woody as readily and can be picked at little larger size, up to 5 inches across.